Magnetostrictive sensors have proven to be effective in measuring force. By measuring the AC resistance or the inductance across a coil of the magnetostrictive sensor it is possible to determine the amount of force subjected to the sensor. Methods of measuring inductance have included the use of resonant circuits and impedance bridges. In a resonant circuit, the resonant frequency depends on inductance; a change in the latter results in changes to the frequency which can be measured with a digital counter or converted to an analog voltage using a frequency to voltage converter circuit. The linearity of this type of measurement depends on the relationship between the inductance and the frequency. In measuring inductance with an impedance bridge, the unknown inductance is placed in the AC equivalent of a Wheatstone bridge which is normally driven by a sinusoidal wave of voltage at a constant frequency. In either a two-leg or four-leg bridge, the bridge is initially balanced by adjusting elements in the bridge. Changes in the known inductance then cause an unbalance and results in a change in AC output voltage. The AC voltage must be converted to a DC voltage by one of a number of standard techniques. By measuring the change in the DC voltage the inductance may be ascertained.
The electrical output of a magnetostrictive sensor includes an in-phase component and a quadrature component and the in-phase component is shifted ninety degrees from the quadrature component. The in-phase component is the real part of the signal and is proportional to the AC resistance of the signal. The quadrature component is the imaginary part of the signal and is proportional to the inductance. A force reading may be ascertained by measuring the AC resistance or the inductance of a coil in the magnetostrictive sensor. A method for measuring the in-phase component and the quadrature component is disclosed in U.S. Pat. No. 4,654,585 (the '585 patent) to Yagi. The '585 patent discloses the utilization of a first and a second phase detector to detect the in-phase component and the quadrature component respectively. The method entails applying a first reference signal at an in-phase angle to the first phase detector to measure the in-phase component and applying a second reference signal shifted by ninety degrees from the reference signal to the second phase detector to measure the quadrature component.
Although the method as disclosed in the '585 patent is useful, there remains an opportunity for an apparatus and a method for detecting the inductance of a magnetostrictive sensor when the magnetostrictive sensor is in the presence of noise introduced by various components in an automobile. Additionally, the measured impedance is a function of temperature and thus varies as temperature changes. Generally, automotive components are exposed to temperatures between −40 C to 150 C. An opportunity exists for providing a calculated inductance or an AC resistance which is independent of temperature to provide for a robust measurement of inductance or an AC resistance which in turn leads to a more accurate reading of force.